Field of the Invention
The present disclosure generally relates to image processing and, more particularly, to an image processing apparatus, an image processing method, and a storage medium for color separation processing.
Description of the Related Art
In an International Color Consortium (ICC) profile that is commonly used in print devices, color material amounts are associated with a device-independent color space such as L*a*b* or XYZ. Examples of color material amounts include cyan (C), magenta (M), yellow (Y), black (K). There are printers with more than ten types of color materials including, in addition to the foregoing color materials, color materials such as red (R), green (G), and blue (B) to reproduce brighter colors and also color materials such as pale cyan (PC), pale magenta (PM), and gray (Gy) to reproduce multi-gradational reproduction.
That is to say, while an image input signal that is conventionally commonly input is three-dimensional or four-dimensional data such as RGB or CMYK, an output of the color separation is data including four to more than ten colors. The dimension number of an output is also high in a case where an input is data that represents colors such as L*a*b* or XYZ as in the case of the ICC profile.
In general, in a case where the dimension number of an output is higher than that of an input, many output solutions exist. For example, in a case of a color separation lookup table (LUT), there exist multiple combinations of CMYK that reproduce one L*a*b*. Concerning existence of the multiple combinations, a well-known color removal (UCR) technique is applied as follows. Suppose that the color material amounts of CMY are (C, M, Y)=(70, 50, 30). FromC′=C−Min(C,M,Y),M′=M−Min(C,M,Y),Y′=Y−Min(C,M,Y), andK=Min(C,M,Y),(C′, M′, Y′, K)=(40, 20, 0, 30) can be obtained. According to a fundamental rule of UCR,(C1, M1, Y1, K1)=(70, 50, 30, 0) (Color material amount 1) and (C2, M2, Y2, K2)=(40, 20, 0, 30) (Color material amount 2) reproduce the same color, although they are different combinations of color material amounts.
The foregoing also applies to a case where colors are associated with color material amounts using a cellular Yule-Nielsen spectral Neugebauer model or the like (Japanese Patent No. 4561483). That is to say, multiple combinations of color material amounts exist for one color.
The multiple combinations of color material amounts can reproduce the same color but have different characteristics in various image quality items such as granularity, which indicates the roughness of an image, color constancy, which indicates a change in color under multiple observation light sources, specular glossiness, and gloss clarity.
The color separation LUT specifies hue information such as L*a*b* or RGB. The color separation LUT stores one combination of color material amounts that are output values with respect to an input value. The technique discussed in Japanese Patent No. 4561483 narrows down the multiple combinations of color material amounts that reproduce the same color to one solution by use of conditions such as a color difference from a target color, color difference under multiple observation light sources, granularity, and a color material amount limiting value that depends on a recording medium. The color separation LUT can store only one combination among the multiple combinations of color material amounts that can reproduce the same color but have different image qualities such as granularity and specular glossiness.
Meanwhile, even if the similar colors of photograph objects are output from a printer, the photograph objects may be diverse such as metal showing a high gloss, fiber showing a low gloss, an object with a rough surface, and an object with a smooth surface (hereinafter, the attribute of an object other than hues, such as gloss and roughness is sometimes referred to as “texture”).
However, since the color separation LUT in a current print system stores only one combination of color material amounts for one hue value, the print system cannot reproduce objects that have the same color but different textures.